1. Field
Example embodiments relate to a semiconductor device and a method of fabricating the same, for example, an image sensor and a method of fabricating the same.
2. Description of the Related Art
Image sensors are semiconductor devices that convert optical images into electrical signals. Referring to FIG. 1, a CMOS image sensor may include a pixel array region that may receive and transfer optical signals, and a logic region that may control or process the optical signals to form an image.
The pixel array region may include unit pixels disposed as an array, and respective unit pixels may include photodiodes for receiving the optical signals and transfer units. The logic region may include a capacitor block for storing analog signals, an Analog-Digital Converter (ADC) block for converting the analog signals into digital signals, a decoder/driver block for selecting a unit pixel, and/or a control register block.
Conventional image sensors may use polysilicon film in a transistor or a capacitor to decrease a noise or a dark defect occurring in the pixel array region. For example, a CMOS image sensor may include a capacitor having electrodes and gate electrodes composed of polysilicon film.
However, despite being doped at a high density, polysilicon film may involve depletion between dissimilar metals. A depletion region may be formed at an interfacial surface between electrodes and a dielectric layer of a capacitor, and may change the capacitance of the capacitor. For example, if depletion occurs, a voltage coefficient of the capacitor may be changed. In turn, if the voltage coefficient of the capacitor is changed, an analog signal may be changed. As a result, resolution of a digital signal from the ACD output unit may be degraded.
Instead of decreasing the noise or the dark defect, the transistors of the logic region may be operated at a higher speed. However, a gate electrode composed of polysilicon film may be unsuitable for a higher operating speed.
Separate fabrication of the logic region and the pixel array region may complicate the fabrication process, and may increase cost.